Method and device for interference cancellation in a CDMA wireless communication system

ABSTRACT

The method of interference cancellation in a CDMA wireless communication system comprises receiving an incident digital signal containing a user signal transmitted on a CDMA user physical channel and an interfering signal, projecting said incident digital signal onto a projection space orthogonal to the space containing said interfering signal, filtering said projected signal with a filter matched to the CDMA user physical channel for detecting the data contained in said user signal.

[0001] The invention relates in general to the field of wirelesscommunication systems, and more particularly to the CDMA systems such asthe different CDMA based mobile radio systems like WCDMA (Wide BandCDMA) and more particularly UTRA-TDD in the downlink situation (UTRA:UMTS Terrestrial Radio Access).

[0002] In a wireless communication system, a central base stationcommunicates with a plurality of remote terminals, such as cellularmobile phones. Frequency-Division Multiple Access (FDMA) andTime-Division Multiple Access (TDMA) are the traditional multiple accessschemes to provide simultaneous services to a number of terminals Thebasic idea behind FDMA and TDMA technics is to slice the availableresource into multiple frequency or time slots, respectively, so thatmultiple terminals can be accomodated without causing interference.

[0003] Contrasting these schemes with separate signals in frequency ortime domains, Code-Division Multiple Access (CDMA) allows multiple usersto share a common frequency and tine channel by using coded modulation.

[0004] More precisely, as it is well-known by the man skilled in theart, a scrambling code which is a long pseudo noise code sequence, isassociated with each base station and permits to distinguish the basestations from each other. Further, an orthogonal code, known by the manskilled in the art under the denomination of OVSF code, is allocated toeach remote terminal (such as cellular mobile phone). All these OVSFcodes are orthogonal with each other, which permits to distinguish aremote terminal from another.

[0005] Before emitting a signal on the wireless transmission channeltowards a remote terminal, the signal has been scrambled and spread bythe base station using the scrambling code of the base station and theOVSF code of the remote terminal.

[0006] Because of possible reflections of the initial transmitted signalon obstacles between the base station and the remote terminal, thewireless transmission channel is in fact a multipath transmissionchannel. As a result, the signal which is received by remote terminalincludes different time shifted versions of the initial transmittedsignal which are the results of the multipath transmissioncharacteristics of the mobile radio channel. Each path introduces adifferent time delay.

[0007] Among the CDMA systems, the CDMA-FDD systems use a differentfrequency for emission and for reception whereas the CDMA-TDD systemsuse a common frequency for emission and reception, but different timedomains for emission and reception.

[0008] The main problem arising from the use of CDMA is the MultipleAccess Interference (MAI) from the users in the cell and the Inter CellInterference (ICI) coming from other cells.

[0009] In recent years, multiuser detection has gained significantnotoriety as a potential advanced technology for the next generation ofCDMA systems. The poor code cross-correlation properties induced by theshort spreading lengths in WCDMA/TDD) lead to severe degradations whenseveral users are transmitting simultaneously, and the conventionalcorrelation receiver appears to be limited. To overcome this majordrawback, several advanced receiver structures have been proposed.

[0010] Unlike the conventional receiver, which treats multiple accessinterference (MAI) as if it were Additive White Gaussian Noise,multiuser receivers treat MAI as additional information to aid indetection. Interference cancellation (IC) is one of several multiuserdetection (MUD) methods to suppress the effects from the MAI andconsequently improve the resulting performance. This in return willincrease the capacity of the communication system.

[0011] Interference cancellation at the mobile terminal is beneficial toperform at high data rates that will be supported by multicodetransmission. Because of multipath propagation, the mobile willexperience multicode interference, from itself as well as from the otherusers. Applying a MUD technique to the downlink is extremely importantsince the system capacity is limited at the downlink. This is furtherenhanced by the higher traffic requirements in the downlink, thepossibility of soft handoff, and the possibility of having antennadiversity.

[0012] Multiuser detection is probably one of the best receptiontechniques) as it removes efficiently the multiple access interference.Among this method, the so-called MMSE (Minimum Mean Square Error) jointdetection, well-known by the man skilled in the art, can be cited.

[0013] However, the major drawback of these conventional multiuserdetection methods lies in its high computational complexity involved byhuge matrix inversions. This extra complexity is not desired at theterminal units, which possess limited battery life and processingcapabilities. Furthermore, such conventional methods assume theknowledge of the spreading codes of the interfering users. However, thisinformation is not always available.

[0014] The invention is intended to provide a solution to this problem.

[0015] One aim of the invention is to provide a blind interferencecancellation without explicit detection of the interfering users.

[0016] Another aim of the invention is to offer a low complexity due toa sliding window technique.

[0017] According to the invention, a projection based approach has beenchosen to reject the multiple access interference, or at least part ofit. The received signal (at the chip level) is made of a useful partsi.e. the data of interest, and the data of other users, which causesinterference, and the noise term.

[0018] The final aim is to reduce as much as possible the interferingpart, without requiring the explicit detection of the correspondingdata. The cancellation process is “blind” in that sense. For thispurpose, the received signal is projected onto the orthogonal complementof the interfering signal space.

[0019] As a result, the projection operation entirely removes theinterfering part from the received vector of chips. But it alsosubtracts part of the useful signal, since the useful and theinterfering spaces are not orthogonal, due to multipath propagation.Nevertheless, it can be shown that this energy loss is largelycompensated by the removed amount of multiple access interference,

[0020] More generally, the invention provides a method of interferencecancellation in a CDMA wireless communication system, which comprisesreceiving an incident digital signal containing a user signaltransmitted on a CDMA user physical channel and an interfering signal,projecting said incident digital signal onto a projection spaceorthogonal to the space containing said interfering signal, filteringsaid projected signal with a filter matched to the CDMA user physicalchannel for detecting the data contained in said user signal.

[0021] According to an embodiment of the invention, said interferingsignal contains interfering information transmitted on at least one CDMAinterfering physical channel; the data transmitted on each CDMA physicalchannel are sent burst by burst, each burst comprising N data symbols;channel coefficients and a specific code are allocated to each physicalCDMA channel during each burst; the projecting phase comprisesdetermining a burst interfering transmission matrix representative ofsaid interfering signal space and containing information on channelcoefficients and specific codes of all the CDMA interfering physicalchannels in use daring said burst transmission, determining saidprojection matrix from said burst interfering transmission matrix, andmultiplying said projection matrix with the N symbols of the receivedburst.

[0022] More particularly, determining said burst interferingtransmission matrix comprises determining a burst transmission matrixcontaining information on channel coefficients and specific codes in useduring said burst transmission, said burst transmission matrixcomprising columns corresponding to said CDMA user physical channel, andremaining columns, and said remaining columns are the columns of saidburst interfering transmission matrix.

[0023] The projection based approach is a powerful tool to performinterference cancellation. Exploiting the inner structure of thetransmission matrix as well as the specificities of the downlinksituation leads to major complexity reductions. Such improvementsbrought by the invention are based on the following facts:

[0024] 1. The limited delay spread: generally TDD systems are adapted tooperate in micro and pico-cellular environments (airports, offices).Such situations will rarely lead to delay spreads greater than 1microsecond (i.e. 4 chips).

[0025] 2. The downlink transmission scenario: the users' data aretransmitted from the base station at the same time. Synchronizationamong users is automatically guaranteed though multipath is stillpresent. Furthermore, for a given receiver, all the data streams fromother users travel across the same wireless channel.

[0026] The above properties induce an almost block diagonal burstinterference transmission matrix with periodically repeating blocks(depending on the spreading factor in use). An embodiment of the presentinvention exploits those specificities, with a sliding window approach,which considers only part of the burst interfering transmission matrixin order to perform the projection operation according to the invention.

[0027] More generally, said projecting and fitting phases can be donestep by step during each reception of a burst at a rate which is equalto the symbol rate or an integer multiple thereof.

[0028] According to an embodiment of the invention, using the fact thatthe burst interference transmission matrix is a block matrix in thesense of the invention as explained more in details thereafter, at eachcurrent step,:

[0029] a current sliding window of at least three consecutive blocks ofthe burst interference transmission matrix is used,

[0030] one projection matrix is computed from said current slidingwindow of at least three consecutive blocks,

[0031] a part of the received burst comprising at least threeconsecutive symbols is multiplied with said one projection matrix, and

[0032] the corresponding projected part of the burst is filtered withsaid filter matched to the CDMA user physical channel.

[0033] Generally, the specific code allocated to a CDMA physical channelcomprises a set of Q code symbols, Q being a spreading factor. The CDMAphysical channels, multiplexed by the base station, are sentsynchronously and travel accross a CDMA wireless channel which is amultipath channel introducing a delay spread which can be smaller thanthe smallest spreading factor. In such a case, the number of consecutiveblocks may be equal to 3, which leads to a very low complexity.

[0034] However, if the delay spread is not smaller than the smallestspreading factor, more than three consecutive blocks, for example four,five or six, can be used leading however to more complexity, Accordingto an embodiment of the invention, in which each computed block of theburst interference transmission matrix comprises a number of columnsequal to the number of CDMA interfering physical channels within theburst, and a number of rows directly depending of the spreading factorsQ of the CDMA interfering physical channels, said projecting phase andsaid filtering phase are done in N steps.

[0035] WCDMA/TDD makes use of the so-called OVSF (Orthogonal VariableSpreading Factor Codes) as channelisation codes. Their two main featuresare their perfect orthogonality, as well as the capability to supportvarious data rates simultaneously. Those spreading codes are generatedaccording to a hierarchical tree. Using codes at a higher level withinthe tree for cancellation purposes allows complexity reductions and alsoenables to suppress several interferers simultaneously.

[0036] In other words, according to an embodiment of the invention inwhich each specific code allocated to each CDMA interfering physicalchannel belongs to a code tree wherein each code, called parent code, ata given level within the tree is used to construct two child codes atthe next level, each child code being obtained by the concatenation ofits parent code, multiplied by +1 or −1, at least two CDMA interferingphysical channels have specific codes associated with spreading factorsand corresponding to a same parent code associated with a parentspreading factor PQ; each computed block of the burst interferencetransmission matrix comprises a number of columns smaller than thenumber of CDMA interfering physical channels within the burst, and anumber of rows directly depending of said parent spreading factor, andsaid projecting phase and said filtering phase are done in pN steps, pbeing equal to Q/PQ.

[0037] The specific code is generally a combination of a spreading codeand a scrambling code. When the respective length of the spreading codeand the scrambling code are equal, each sliding window contains the sameblocks.

[0038] However, when the length of the spreading code is smaller thanthe length of the scrambling code, at least two consecutive slidingwindows contain different blocks. In other words, in such a case, thisinduces a periodically time varying spreading codes and blocks withinthe transmission matrix also alternate periodically.

[0039] The method according to the invention is advantageously used whenthe CDMA wireless communication system is an UTRA-TDD wirelesscommunication system and when the incident signal is emitted by a basestation, i.e. in the downlink situation.

[0040] The invention proposes also an interference cancellation devicefor a CDMA wireless communication system, comprising:

[0041] reception means for receiving an incident digital signalcontaining a user signal transmitted on a CDMA user physical channel andan interfering signal,

[0042] preprocessing means for projecting said incident digital signalonto a projection space orthogonal to the space containing saidinterfering signal, and

[0043] a Rake receiver connected to the output of the preprocessingmeans, and matched to the CDMA user physical channel for detecting thedata contained in said user signal.

[0044] According to an embodiment of the invention, said interferingsignal contains interfering information transmitted on at least one CDMAinterfering physical channel; the data transmitted on each CDMA physicalchannel are sent burst by burst, each burst comprising N data symbols;channel coefficients and a specific code are allocated to each physicalCDMA channel during each burst; the preprocessing means comprises:

[0045] first calculation means for determining a burst interferingtransmission matrix representative of said interfering signal space andcontaining information on channel coefficients and specific codes of allthe CDMA interfering physical channels in use during said bursttransmission,

[0046] second calculation means for determining said projection matrixfrom said burst interfering transmission matrix, and

[0047] multiplication means for multiplying said projection matrix withthe N symbols of the received burst.

[0048] According to an embodiment of the invention, said first iscalculation means comprises means for determining a burst transmissionmatrix containing information on channel coefficients and specific codesin use during said burst transmission, said burst transmission matrixcomprising columns corresponding to said CDMA user physical channel, andremaining columns; said remaining columns are the columns of said burstinterfering transmission matrix.

[0049] According to an embodiment of the invention, said preprocessingmeans and said Rake receiver are adapted to perform the projecting andfiltering phases step by step during each reception of a burst at a ratewhich is equal to the symbol rate or an integer multiple thereof.

[0050] According to an embodiment of the invention, said burstinterference transmission matrix is a block matrix, and at each currentstep:

[0051] the preprocessing means is adapted for using a current slidingwindow of at least three consecutive blocks of the burst interferencetransmission matrix, for computing one projection matrix from saidcurrent sliding window of at least three consecutive blocks, formultiplying a part of the received burst comprising three consecutivesymbols with said one projection matrix, and

[0052] the Rake receiver is adapted for filtering the correspondingprojected part of the burst.

[0053] According to an embodiment of the invention, the specific codeallocated to a CDMA physical channel comprises a set of Q code symbols,Q being a spreading factor; the CDMA physical channels are sent across aCDMA wireless channel which is a multipath channel introducing a delayspread smaller than the smallest spreading factor, and the number ofconsecutive blocks is equal to three.

[0054] According to an embodiment of the invention, each computed blockof the burst interference transmission matrix comprises a number ofcolumns equal to the number of CDMA interfering physical channels withinthe burst, and a number of rows directly depending of the spreadingfactors Q of the CDMA interfering physical channels, and saidpreprocessing means and said Rake receiver are adapted to perform theprojecting and filtering phases in N steps.

[0055] According to an embodiment of the invention, each specific codeallocated to each CDMA interfering physical channel belongs to a codetree wherein each code, called parent code, at a given level within thetree is used to construct two child codes at the next level, each childcode being obtained by the concatenation of its parent code, multipliedby +1 or −1; at least two CDMA interfering physical channels havespecific codes associated with spreading factors and corresponding to asame parent code associated with a parent spreading factor PQ; eachblock of the burst interference transmission matrix comprises a numberof columns smaller than the number of CDMA interfering physical channelswithin the burst, and a number of rows directly depending of said parentspreading factor; and said preprocessing means and said Rake receiverare adapted to perform the projecting and filtering phases in pN steps,p being equal to Q/PQ.

[0056] According to an embodiment of the invention, the specific code isthe combination of a spreading code and a scrambling code; therespective lengths of the spreading code and the scrambling code areequal, and each sliding window contains the same blocks.

[0057] According to an embodiment of the invention, the specific code isthe combination of a spreading code and a scrambling code; the length ofthe spreading code is smaller than the length of the scrambling code,and at least two consecutive sliding windows contain different blocks.

[0058] The invention proposes also a receiver, in particular a cellularmobile phone, of a CDMA wireless communication system, comprising aninterference cancellation device as defined above.

[0059] The invention proposes also a base station of a CDMA wirelesscommunication station comprising an interference cancellation device asdefined above.

[0060] Other advantages and features of the invention will appear inexamining the detailed description of embodiments, these being in no waylimited, and of the appendent drawings in which:

[0061]FIG. 1 illustrates very diagrammatically a cellular mobile phoneaccording to the invention incorporating an interference cancellationdevice according to the invention;

[0062]FIG. 2 illustrates a code-tree for generation of OrthogonalVariable Spreading Factor (OVSF) codes for channelisation operation;

[0063]FIG. 3 illustrates a physical channel signal format;

[0064]FIG. 4 illustrates a timeslot format;

[0065]FIG. 5 illustrates a downlink transmission situation;

[0066]FIG. 6 illustrates a structure of the A matrix (in a user/symbolform);

[0067]FIG. 7 illustrates a structure of the A_(i) blocks of the A matrixof FIG. 6;

[0068]FIG. 8 depicts a geometrical interpretation of the invention;

[0069]FIG. 9 illustrates diagrammatically a projection basedinterference cancellation process according to the invention;

[0070]FIG. 10 illustrates a symbol by symbol approach according to theinvention; and

[0071]FIG. 11 illustrates a symbol by symbol cancellation processaccording to the invention.

[0072] In FIG. 1, the reference TP denotes a remote terminal such as acellular mobile phone which is in communication with a base station BS1.In this embodiment, the wireless communication system is an UTRA-TDDsystem, and only the downlink situation will be described.

[0073] The mobile phone TP comprises, conventionally, an analog radiofrequency front end stage ERF connected to an antenna ANT2 for receivingan input signal ISG.

[0074] Conventionally, the stage ERF comprises a low noise amplifier LNAand two processing channels including mixers and conventional filtersand amplifiers (not shown). The two mixers receive respectively from aphase locked loop PLL two signals, having mutually a phase difference of90°. After frequency transposition in the mixers, the two processingchannels define respectively two streams I and Q as it is well known bythe man skilled in the art. After digital conversion intoanalog-to-digital converters A/D, the two digital streams I and Q aredelivered to a digital processing stage ETN.

[0075] This digital stage ETN comprises preprocessing means PPM followedby a Rake receiver RR followed by conventional demapping means MP(demodulation means) which perform the demodulation of the constellationdelivered by the Rake receiver. The stage ETN comprises alsoconventionally a channel decoder and a source decoder SD which performsa source decoding well-known by the man skilled in the art.

[0076] Preprocessing means PPM and the Rake receiver RR form together aninterference cancellation device ICD according to the invention, TheRake receiver is a conventional one. An example of a Rake receiver maybe found in EP 1175019. The Rake receiver is adapted to perform a singleuser matched filtering. More precisely, each finger of the Rake receiveracts as a correlator matched to a delay on which significant energyarrives; symbols at the output of each finger are phase-adjusted andthen combined; channel estimation based on known pilot symbols enablesradio channel tracking and fingers parameters update.

[0077] At last, as it is also well-known by the man skilled in the art,the phase locked loop PLL is controlled by an automatic frequencycontrol algorithm incorporated in a processor of the stage ETN.

[0078] The received signal ISG results from the transmission of aninitial signal by the antenna ANT1 of the base station BS1 on amultipath channel transmission MPC. In the present embodiment, it isassumed that the mobile phone TP receives a signal from base station BS1only. But of course, the received signal ISG could also result from thetransmission of initial signals respectively emitted by severaldifferent base stations BS1 and BS2.

[0079] Because of possible reflections of the signal on obstacleslocated between the base station BS1 and the mobile phone TP, thetransmission channel MPC comprises several different transmission paths(here three paths P1, P2, P3 are shown).

[0080] As it is well known by the man skilled in the art, beforetransmission through the antenna ANT1, the initial signal containing thedata (symbols) is scrambled and spread by the processing means of thebase station BS1, by using the scrambling code of the base station andthe orthogonal code of the phone TP.

[0081] Since CDMA is of concern, the data symbol sequence modulated by aQPSK data modulation is spread with a real spreading code. In WCDMA/TDDand more particularly in UTRA-TDD, the utilized codes are the so-calledOVSF (Orthogonal Variable Spreading Factor) codes, which will furtherallow to mix different data rates within one time slot while preservingthe orthogonality.

[0082] The elements c_(q) ^((k)); k=1, . . . , K; q=1, . . . , Q_(k); ofthe real valued channelisation codes c^((k))=(c₁ ^((k)), c₂ ^((k)), . .. , c_(Q) ^((k))), k=1, . . . , K; shall be taken from the setV_(c)={1,−1}. k is the user index and Q_(k) denotes the spreading factorof the k^(th) user. The OVSF codes can be defined using the code tree ofFIG. 2.

[0083] Each level in the code tree defines a spreading factor indicatedby the value of Q in the figure. All codes within the code tree cannotbe used simultaneously in a given timeslot. A code can be used in atimeslot if and only if no other code on the path from the specific codeto the root of the tree or in the sub-tree below the specific code isused in this timeslot. This means that the number of available codes ina slot is not fixed but depends on the rate and spreading factor of eachphysical channel. The spreading factor goes up to Q_(MAX)=16.

[0084] The orthogonality property leads to a perfect multiuserinterference cancellation, within one propagation path, since codesequences are synchronized in this case.

[0085] The spreading of data by a real valued channelisation codec^((k)) of length Q_(k) is followed by a cell specific complexscrambling sequence i=(i ₁, i ₂, . . . , i ₁₆). The elements i _(i);i=1,. . . , 16 of the complex valued scrambling codes shall be taken fromthe complex set V _(v)={1, j,−1, −j}.

[0086] Consequently, the symbols are transformed into chips having apredetermined length Tc (for example equal to 260 ns), corresponding toa predetermined chip rate (of 3.84 Mcps for example). The chip rate isgreater than the data or symbol rate. For example, one symbol can betransformed into 4 to 256 chips.

[0087] The initial signal constituted of chips is then filtered in amatched filter before analog conversion and transmission through antennaANT1.

[0088] After analog-to-digital conversion in the A/D converters of thephone TP, the signal (complex signal constituted of the two streams Iand Q) is a digital scrambled and spreaded signal constituted of chips,oversampled with an oversampling factor Ns (Ns=4 for example). Thisdigital signal DSN includes delayed versions of the initial scrambledand spread signal transmitted by the base station.

[0089] All physical channels take a structure of radio frames, andtimeslots. Timeslots add a TDMA component, which allows to separatedifferent users in the time in addition to the code domain. Each frameincludes 15 timeslots. Bach timeslot is a sequence of 2560 chips. Thestructure appears on FIG. 3.

[0090] Each timeslot comprises two data parts, separated by a midamble,which acts as a training sequence (FIG. 4). GP denotes the guard period,which is needed to compensate the propagation delay. TFCI stands forTransport Format Combination Indicator and is used to inform thereceiver about the instantaneous parameters of the different transportchannels multiplexed onto one physical channel. If power control isapplied, the TPC (Transmit Power Control) field carries commands toadjust the transmission power. A physical channel is defined by carrierfrequency, timeslot, channelisation code (or spreading code), bursttype, repetition period, superframe offset and repetition length.

[0091] In the following, the term “user” denotes more generally a WCDMAphysical channel with its own spreading code.

[0092] Further, as illustrated on FIG. 5, a typical downlink situationhas been considered. The base station BS1 transmits data to K userswithin the serving cell. Since CDMA is of concern, the physical channels(users) are multiplexed by the base station and sent synchronouslywithin a burst and travel accross the same wireless channel up to theuser of interest (user 1 in the present case).

[0093] Further, in the, following, the data symbol vector${\overset{\_}{d}}^{(k)} = \begin{bmatrix}d_{1}^{(k)} \\\vdots \\d_{N}^{(k)}\end{bmatrix}$

[0094] represents the data sent by user k during one burst (N QPSKsymbols). The midamble is not considered and the N symbols arecontituted by the N/2 symbols of each data part.

[0095] The code of the k^(th) user combines spreading and scrambling,and is a set of Q complex QPSK symbols, {c₁ ^((k)), . . . , c _(O)^((k))}, where Q is the spreading length.

[0096] The channel impulse response of the wireless channel is modeledfor each user by the vector ${\overset{\_}{h}}^{(k)} = {\begin{bmatrix}h_{1}^{(k)} \\\vdots \\h_{W}^{(k)}\end{bmatrix}.}$

[0097] The channel length is equal to W chips. The superscript k couldin fact be removed, since the downlink situation is addressed. Thechannel is assumed to remain unchanged over one burst duration, which isgenerally true for moderate terminal velocities.

[0098] The response of the wireless channel to the k^(th) user'sspreading sequence is: b^((k))=c^((k))*h^((k)), which can be written ina matrix vector form: $\begin{bmatrix}b_{1}^{(k)} \\b_{2}^{(k)} \\b_{3}^{(k)} \\\vdots \\\vdots \\\vdots \\b_{Q + W - 1}^{(k)}\end{bmatrix} = {\begin{bmatrix}c_{1}^{(k)} & 0 & 0 & 0 \\c_{2}^{(k)} & c_{1}^{(k)} & 0 & 0 \\\vdots & c_{2}^{(k)} & ⋰ & 0 \\c_{Q}^{(k)} & \vdots & \vdots & c_{1}^{(k)} \\0 & c_{Q}^{(k)} & \vdots & c_{2}^{(k)} \\0 & 0 & ⋰ & \vdots \\0 & 0 & 0 & c_{Q}^{(k)}\end{bmatrix} \cdot \begin{bmatrix}h_{1}^{(k)} \\h_{2}^{(k)} \\\vdots \\h_{W}^{(k)}\end{bmatrix}}$

[0099] The data symbol d_(j) ^((k)) (scalar quantity) is sent throughthe wireless channel as follows: (d_(j) ^((k))·c^((k)))*h^((k))=d_(j)^((k))·(c^((k))*h^((k)))=d_(j) ^((k))·b^((k))

[0100] The transmission of the whole data burst can be expressed usingthe A matrix defined below, for K=2 users. $\begin{bmatrix}r_{1} \\r_{2} \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\r_{{NQ} + W - 1}\end{bmatrix} = {{\left\lbrack \begin{matrix}b_{1}^{(1)} & 0 & 0 & 0 \\b_{2}^{(1)} & 0 & 0 & 0 \\\vdots & 0 & 0 & 0 \\b_{Q + 1}^{(1)} & b_{1}^{(1)} & 0 & 0 \\\vdots & b_{2}^{(1)} & 0 & 0 \\b_{Q + W - 1}^{(1)} & \vdots & 0 & 0 \\0 & \vdots & b_{Q + W - 1}^{(1)} & 0 \\0 & \vdots & \vdots & 0 \\0 & b_{Q + W - 1}^{(1)} & \vdots & 0 \\0 & 0 & \vdots & b_{t}^{(1)} \\0 & 0 & \vdots & \vdots \\0 & 0 & b_{Q + W - 1}^{(1)} & \vdots \\0 & 0 & 0 & \vdots \\0 & 0 & 0 & \vdots \\0 & 0 & 0 & b_{Q + W - 1}^{(1)}\end{matrix} \middle| \begin{matrix}b_{1}^{(2)} & 0 & 0 & 0 \\b_{2}^{(2)} & 0 & 0 & 0 \\\vdots & 0 & 0 & 0 \\b_{Q + 1}^{(2)} & b_{1}^{(2)} & 0 & 0 \\\vdots & \vdots & 0 & 0 \\b_{Q + W - 1}^{(2)} & \vdots & 0 & 0 \\0 & \vdots & b_{1}^{(2)} & 0 \\0 & \vdots & \vdots & 0 \\0 & b_{Q + W - 1}^{(2)} & \vdots & 0 \\0 & 0 & \vdots & b_{1}^{(2)} \\0 & 0 & \vdots & \vdots \\0 & 0 & b_{Q + W - 1}^{(2)} & \vdots \\0 & 0 & 0 & \vdots \\0 & 0 & 0 & \vdots \\0 & 0 & 0 & b_{Q + W - 1}^{(2)}\end{matrix} \right\rbrack \cdot \begin{bmatrix}d_{1}^{(1)} \\\vdots \\\vdots \\\frac{d_{N}^{(1)}}{d_{1}^{(2)}} \\\vdots \\\vdots \\d_{N}^{(2)}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2} \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\n_{{NQ} + W - 1}\end{bmatrix}}$

[0101] The noise vector is assumed to be uncorrelated Gaussian noise.The received burst can then be written as:$r = {{{A \cdot d} + n} = {{\left\lbrack {A_{1},\quad A_{2}} \right\rbrack \cdot \begin{bmatrix}d^{(1)} \\d^{(2)}\end{bmatrix}} + n}}$

[0102] where A₁ and A₂ denote the-contributions of users 1 and 2respectively. The situation can easily be generalized to K simultaneoususers, with A and d given below: $\begin{matrix}{A = {{\left\lbrack {A_{1},\quad A_{2},\quad \ldots \quad,\quad A_{K}} \right\rbrack \quad d} = \begin{bmatrix}d^{(1)} \\\vdots \\d^{(K)}\end{bmatrix}}} \\{r = {{{A \cdot d} + n} = {{\left\lbrack {A_{1},\quad \ldots \quad,\quad A_{K}} \right\rbrack \cdot \begin{bmatrix}d^{(1)} \\\vdots \\d^{(K)}\end{bmatrix}} + n}}}\end{matrix}$

[0103] Another useful representation for the A burst transmission matrixis the one where the user contributions are gathered inside the sameblock. Then, adjacent blocks correspond to different symbols. This ofcourse requires a reordering within the data vector d.

[0104] The structure of such a burst transmission matrix A is definedbelow, for K=2 users: $\begin{bmatrix}r_{1} \\r_{2} \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\r_{{NQ} + W - 1}\end{bmatrix} = {{\begin{bmatrix}b_{1}^{(1)} & b_{1}^{(2)} & 0 & 0 & 0 & 0 & 0 & 0 \\b_{2}^{(1)} & b_{2}^{(2)} & 0 & 0 & 0 & 0 & 0 & 0 \\\vdots & \vdots & 0 & 0 & 0 & 0 & 0 & 0 \\b_{Q + 1}^{(1)} & b_{Q + 1}^{(2)} & b_{1}^{(1)} & b_{1}^{(2)} & 0 & 0 & 0 & 0 \\\vdots & \vdots & b_{2}^{(1)} & b_{2}^{(2)} & 0 & 0 & 0 & 0 \\b_{Q + W - 1}^{(1)} & b_{Q + W - 1}^{(2)} & \vdots & \vdots & 0 & 0 & 0 & 0 \\0 & 0 & b_{Q + 1}^{(1)} & b_{Q + 1}^{(2)} & b_{1}^{(1)} & b_{1}^{(2)} & 0 & 0 \\0 & 0 & \vdots & \vdots & b_{2}^{(1)} & b_{2}^{(2)} & 0 & 0 \\0 & 0 & b_{Q + W - 1}^{(1)} & b_{Q + W - 1}^{(2)} & \vdots & \vdots & 0 & 0 \\0 & 0 & 0 & 0 & b_{Q + 1}^{(1)} & b_{Q + 1}^{(2)} & b_{1}^{(1)} & b_{1}^{(2)} \\0 & 0 & 0 & 0 & \vdots & \vdots & b_{2}^{(1)} & b_{2}^{(2)} \\0 & 0 & 0 & 0 & b_{Q + W - 1}^{(2)} & b_{Q + W - 1}^{(2)} & \vdots & \vdots \\0 & 0 & 0 & 0 & 0 & 0 & b_{Q + 1}^{(1)} & b_{Q + 1}^{(2)} \\0 & 0 & 0 & 0 & 0 & 0 & \vdots & \vdots \\0 & 0 & 0 & 0 & 0 & 0 & b_{Q + W - 1}^{(1)} & b_{Q + W - 1}^{(2)}\end{bmatrix} \cdot \begin{bmatrix}d_{1}^{(1)} \\d_{1}^{(2)} \\d_{2}^{(1)} \\d_{2}^{(2)} \\\vdots \\\vdots \\d_{N}^{(1)} \\d_{N}^{(2)}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2} \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\\vdots \\n_{{NQ} + W - 1}\end{bmatrix}}$

[0105] For K users, it can be generalized as illustrated on FIG. 6 whereA_(i) is illustrated on FIG. 7.

[0106] A is a not a block diagonal matrix (block are placed every Qrows), but the blocks are overlapping together, due to multipathpropagation, which leads to inter-symbol interference. However, in thesense of the present invention, A is nevertheless called a “blockmatrix”.

[0107] Since the wireless channel is assumed to remain constant over oneburst duration, and since the spreading codes do not change from onesymbol to another, the blocks A_(i) within the A burst transmissionmatrix are all identical.

[0108] The main idea to reject the multiple access interference, or atleast part of it, is to use a projection based approach. The receivedsignal r (at the chip level) is made of a useful part r₁, i.e. the dataof interest (i.e. user 1's data), the data of other users r₁, whichcauses interference, and the noise term n:

r=r ₁ +r _(i) +n

[0109] The final aim is to reduce as much as possible the r_(i)component, without requiring the explicit detection of the correspondingdata. The cancellation process is “blind” in that sense. This can beperformed following the procedure below:

[0110] 1. Find a basis which spans the interfering signal space

[0111] 2. Build a basis that spans the space orthogonal to the one in 1

[0112] 3. Project the received signal onto the space defined in 2

[0113] 4. Feed the resulting signal to a conventional Rake receiver

[0114] 5, Detect the data of user 1

[0115] The geometrical interpretation is depicted in FIG. 8.

[0116] 1) The useful signal lies in a vector space spanned by thecolumns of A corresponding to user 1 (those which contain b⁽¹⁾).

[0117] 2) The interfering signal space is spanned by the remainingcolumns of A (containing b⁽²⁾, . . . , b^((k))).

[0118] The projection method entirely removes the interfering part r_(i)from the received vector of chips r. But it also subtracts part of theuseful signal r₁, since the useful and the interfering spaces are notorthogonal, due to multipath propagation. Nevertheless, this energy lossis largely compensated by the removed amount of multiple accessinterference.

[0119] Then a projection matrix M is built, which spans the orthogonalcomplement of the interfering signal space, in the following way:

M=I−A _(I)(A _(I) ^(H) A _(I))⁻¹ A _(I) ^(H)

[0120] where the columns of the interfering burst transmission matrix A₁span the interfering space. The notation “H” means “hermitiantranspose”.

[0121] Then, the received burst r is processed as follows: {overscore(r)}_(c)=M·{overscore (r)}, where r_(c) is the cancelled or projectedvector of chips. As mentioned before, the multiple access interferenceis perfectly removed, provided that a description of the interferingsignal space is known (i.e. the knowledge of the codes in use and of thechannel impulse response), which is the case in such a wirelesscommunication system.

[0122] The cancellation process is “blind” in the sense that theinterfering data is not explicitly detected.

[0123] To finish with, the cancelled vector is passed through the Rakereceiver RR matched to user 1, in order to estimate its data symbols:

{overscore ({acute over (d)})} ₁ =A ₁ ^(H) ·{overscore (r)} _(c)

[0124] The whole cancellation procedure is summarized in FIG. 9.

[0125] As such, this projection based approach can be too complex inorder to be implemented in a real-time fashion in some types of mobileterminals, as cellular mobile phones.

[0126] Thus, according to a preferred embodiment of the invention, asymbol by symbol processing technique can provide with a majorcomplexity reduction.

[0127] More precisely, the previous projection matrix computationconsidered the complete interfering burst transmission matrix A_(i),i.e. over the whole burst duration. After a closer look at the innerstructure of A_(i), such an approach can be avoided, given the fact thatthe delay spread introduced by the wireless channel is limited. It issufficient to consider only three consecutive blocks within A_(i),provided that the delay spread remains smaller than the spreading factorQ.

[0128] Therefore, the projection matrix M is computed from 3 consecutiveblocks of the interfering burst transmission matrix, where as usual,only columns related to the interferers are taken into account. Thecancellation is now done in N steps (N=number of symbols), one symbolafter another. Then, the cancelled symbol is passed through the Rakereceiver matched to user 1. The process is illustrated in 10 and FIG.11.

[0129] The man skilled in the art can note that, at each step, oneprojection matrix is computed from three consecutive blocks and itdescribes the orthogonal subspace to the signal space spaned by threeconsecutive interfering symbols. With such sliding window processing,the complexity is reduced since the matrix to be inverted has a smallersize than the one to be inverted when the whole burst is considered.

[0130] Generally speaking, each computed block of the burst interferencetransmission matrix comprises a number of columns equal to the number ofCDMA interfering physical channels (interfering users) within the burst,and a number of rows directly depending from the spreading factors Q ofthe interfering users (and depending also on the length considered forthe channel impulse responses).

[0131] Interference cancellation as previously considered was done atthe interfering symbol rate, i.e. said projection phase and saidfiltering phase were done in N steps.

[0132] That means that the specific codes allocated to each CDMAinterfering physical channels have directly been used in order tocompute a projection matrix.

[0133] However, it is possible again to reduce the complexity by higherlevel projections.

[0134] In that sense, the specific structure of the OVSF code tree asillustrated in FIG. 2, can be exploited given the following facts:

[0135] 1) Each code at a given level (spreading factor Q₁) within thetree is used to construct 2 child codes at the next level (spreadingfactor Q₂=2*Q₁).

[0136] 2) Each child code is obtained by the concatenation of its parentcode, multiplied by +1 or −1.

[0137] And, considering now that at least two CDMA interfering physicalchannels have specific codes associated with spreading factors andcorresponding to a same parent code associated with a parent spreadingfactor PQ, each computed block of the burst interference transmissionmatrix comprises a number of rows directly depending on said parentspreading factor. And, in that case, said projecting phase and saidfiltering phase are done in pN steps, p being equal to Q/PQ.

[0138] In that case, said projecting and filtering phases are done stepby step during each reception of a burst at a rate which is equal to aninteger multiple of the symbol rate.

[0139] More precisely and for example, one interferer with Q=16 can beseen as a fictitious double rate user with Q=8, with twice the number ofsymbols. The spreading factor used for cancellation purposes can thus beset to 8, using the parent code of the interfering code. Furthermore, iftwo interferers (with spreading factor Q) have the same parent code,they can be both cancelled using a single code with Q/2. Hence thenumber of interfering codes is artificially reduced by 2. Similarly, upto four users with Q=16 can be cancelled using the generating code withspreading factor 4.

[0140] Generally, the specific code is the combination of a spreadingcode and of a scrambling code. More precisely, the scrambling operationcomes after spreading and consists in a chip by chip multiplication ofthe spread data with a scrambling code.

[0141] When the respective lengths of the spreading code and thescrambling code are equal, each sliding window, in the sliding windowapproach contains the same blocks. In other words, the blocks areidentical from one sliding window to another.

[0142] However, a problem arises when spreading and scrambling lengthsdiffer. This occurs in particular in the UTRA-TDD system, in which thescrambling length is equal to 16 chips whereas the spreading factors canbe equal to 1 or 16. Spreading factors equal to 2, 4, or 8 can also beused theoretically but they are for the time being not used practicallyin a TDD system.

[0143] In that case, at least two consecutive sliding windows containdifferent blocks.

[0144] More precisely, the fact that the lengths of the scrambling codeand of the spreading code are different, induces a periodically timevarying spreading code and blocks within the burst interferingtransmission matrix, also alternate periodically. Thus, with the slidingwindow approach, two different projection matrices have to be computedat each step when the spreading factor is equal to 8, whereas fourprojection matrices have to be computered when the spreading factor isequal to 4, and so on (16 projection matrices have to be computed whenthe spreading factor is equal to 1 ).

[0145] While the above description contains certain specifications, thisshould not construe as limitations on the scope of the invention butrather an exemplification of one preferred embodiment and applicationthereof. It will be apparent to those skilled in the art that variousmodifications can be made to the invention without departing from thescope or spirit of the invention and it is intended that the presentinvention covers modifications and variations of the interferencecancellation method and device provided they come in the scope of theappended claims and their equivalence.

[0146] For example, the invention can also be deployed in a FDD basedsystem and for the future High Speed Packet Down Link (HSPDA) mode ofthe 3GPP standard.

[0147] Further, although only the downlink situation has been describedin details, the invention can be applied to the uplink situation aswell, using different channels impulse responses for the users.

[0148] Further, this projection based approach according to theinvention can also be employed to suppress interfering signals fromother cells (other base stations). However, some modifications areneeded, which can be easily be made by the man skilled in the art. Forexample a basis for these interfering signals must first be built.

1. Method of interference cancellation in a CDMA wireless communicationsystem, characterized by the fact that it comprises receiving anincident digital signal containing a user signal transmitted on a CDMAuser physical channel and an interfering signal, projecting saidincident digital signal onto a projection space orthogonal to the spacecontaining said interfering signal, filtering said projected signal witha filter matched to the CDMA user physical channel for detecting thedata contained in said user signal.
 2. Method according to claim 1,characterized by the fact that said interfering signal containsinterfering information transmitted on at least one CDMA interferingphysical channel, by the fact that the data transmitted on each CDMAphysical channel are sent burst by burst, each burst comprising N datasymbols, by the fact that channel coefficients and a specific code areallocated to each physical CDMA channel during each burst, by the factthat the projecting phase comprises determining a burst interferingtransmission matrix representative of said interfering signal space andcontaining information on channel coefficients and specific codes of allthe CDMA interfering physical channels in use during said bursttransmission, determining said projection matrix from said burstinterfering transmission matrix, and multiplying said projection matrixwith the N symbols of the received burst.
 3. Method according to claim2, characterized by the fact that determining said burst interferingtransmission matrix comprises determining a burst transmission matrixcontaining information on channel coefficients and specific codes in useduring said burst transmission, said burst transmission matrixcomprising columns corresponding to said CDMA user physical channel, andremaining columns, and by the fact that said remaining columns are thecolumns of said burst interfering transmission matrix.
 4. Methodaccording to claim 2 characterized by the fact that said projecting andfiltering phases are done step by step during each reception of a burstat a rate which is equal to the symbol rate or an integer multiplethereof.
 5. Method according to claim 4, characterized by the fact thatthat said burst interference transmission matrix is a block matrix, bythe fact that at each current step: a current sliding window of at leastthree consecutive blocks of the burst interference transmission matrixis used, one projection matrix is computed from said current slidingwindow of at least three consecutive blocks, a part of the receivedburst comprising at least three consecutive symbols is multiplied withsaid one projection matrix, and the corresponding projected part of theburst is filtered with said filter matched to the CDMA user physicalchannel.
 6. Method according to claim 5, characterized by the fact thatthe specific code allocated to a CDMA physical channel comprises a setof Q code symbols, Q being a spreading factor, by the fact that the CDMAphysical channels are sent across a CDMA wireless channel which is amultipath channel introducing a delay spread smaller than the smallestspreading factor, and by the fact that the number of consecutive blocksis equal to three.
 7. Method according to claim 5 characterized by thefact that each computed block of the burst interference transmissionmatrix comprises a number of columns equal to the number of CDMAinterfering physical channels within the burst, and a number of rowsdirectly depending of the spreading factors Q of the CDMA interferingphysical channels, and by the fact that said projecting phase and saidfiltering phase are done in N steps.
 8. Method according to claim 5characterized by the fact that each specific code allocated to each CDMAinterfering physical channel belongs to a code tree wherein each code,called parent code, at a given level within the tree is used toconstruct two child codes at the next level, each child code beingobtained by the concatenation of its parent code, multiplied by +1 or−1, by the fact that at least two CDMA interfering physical channelshave specific codes associated with spreading factors and correspondingto a same parent code associated with a parent spreading factor PQ, bythe fact that each computed block of the burst interference transmissionmatrix comprises a number of columns smaller than the number of CDMAinterfering physical channels within the burst, and a number of rowsdirectly depending of said parent spreading factor, and by the fact thatsaid projecting phase and said filtering phase are done in pN steps, pbeing equal to Q/PQ.
 9. Method according to any one of claims 5characterized by the fact that the specific code is the combination of aspreading code and a scrambling code, by the fact that the respectivelengths of the spreading code and the scrambling code are equal, and bythe fact that each sliding window contains the same blocks.
 10. Methodaccording to any one of claims 5 characterized by the fact that thespecific code is the combination of a spreading code and a scramblingcode, by the fact that the length of the spreading code is smaller thanthe length of the scrambling code, and by the fact that at least twoconsecutive sliding windows contain different blocks.
 11. Methodaccording to any one of claims 1 characterized by the fact that the CDMAwireless communication system is an UTRA-TDD wireless communicationsystem, and by the fact that the incident signal is emitted by a basestation.
 12. Interference cancellation device for a CDMA wirelesscommunication system, characterized by the fact that it comprisesreception means for receiving an incident digital signal containing auser signal transmitted on a CDMA user physical channel and aninterfering signal, preprocessing means for projecting said incidentdigital signal onto a projection space orthogonal to the spacecontaining said interfering signal, and a flake receiver connected tothe output of the preprocessing means, and matched to the CDMA userphysical channel for detecting the data contained in said user signal.13. Device according to claim 12, characterized by the fact that saidinterfering signal contains interfering information transmitted on atleast one CDMA interfering physical channel, by the fact that the datatransmitted on each CDMA physical channel are sent burst by burst, eachburst comprising N data symbols, by the fact that channel coefficientsand a specific code are allocated to each physical CDMA channel duringeach burst, by the fact that the preprocessing means comprises: firstcalculation means for determining a burst interfering transmissionmatrix representative of said interfering signal space and containinginformation on channel coefficients and specific codes of all the CDMAinterfering physical channels in use during said burst transmission,second calculation means for determining said projection matrix fromsaid burst interfering transmission matrix, and multiplication means formultiplying said projection matrix with the N symbols of the receivedburst.
 14. Device according to claim 13, characterized by the fact thatsaid first calculation means comprises means for determining a bursttransmission matrix containing information on channel coefficients andspecific codes in use during said burst transmission, said bursttransmission matrix comprising columns corresponding to said CDMA userphysical channel, and remaining columns, and by the fact that saidremaining columns are the columns of said burst interfering transmissionmatrix.
 15. Device according to claim 13 characterized by the fact thatsaid preprocessing means and said Rake receiver are adapted to performthe projecting and filtering phases step by step during each receptionof a burst at a rate which is equal to the symbol rate or an integermultiple thereof.
 16. Device according to claim 15, characterized by thefact that that said burst interference transmission matrix is a blockmatrix, by the fact that at each current step: the preprocessing meansis adapted for using a current sliding window of at least threeconsecutive blocks of the burst interference transmission matrix, forcomputing one projection matrix from said current sliding window of atleast three consecutive blocks, for mutiplying a part of the receivedburst comprising three consecutive symbols with said one projectionmatrix, and the Rake receiver is adapted for filtering the correspondingprojected part of the burst.
 17. Device according to claim 16,characterized by the fact that the specific code allocated to a CDMAphysical channel comprises a set of Q code symbols, Q being a spreadingfactor, by the fact that the CDMA physical channels are sent across aCDMA wireless channel which is a multipath channel introducing a delayspread smaller than the smallest spreading factor, and by the fact thatthe number of consecutive blocks is equal to three.
 18. Device accordingto claim 16 characterized by the fact that each computed block of theburst interference transmission matrix comprises a number of columnsequal to the number of CDMA interfering physical channels within theburst, and a number of rows directly depending of the spreading factorsQ of the CDMA interfering physical channels, and by the fact that saidpreprocessing means and said Rake receiver are adapted to perform theprojecting and filtering phases in N steps.
 19. Device according toclaim 16 characterized by the fact that each specific code allocated toeach CDMA interfering physical channel belongs to a code tree whereineach code, called parent code, at a given level within the tree is usedto construct two child codes at the next level, each child code beingobtained by the concatenation of its parent code, multiplied by +1 or−1, by the fact that at least two CDMA interfering physical channelshave specific codes associated with spreading factors and correspondingto a same parent code associated with a parent spreading factor PQ, bythe fact that each block of the burst interference transmission matrixcomprises a number of columns smaller than the number of CDMAinterfering physical channels within the burst, and a number of rowsdirectly depending of said parent spreading factor, and by the fact saidpreprocessing means and said Rake receiver are adapted to perform theprojecting and filtering phases in pN steps, p being equal to Q/PQ. 20.Device according to any one of claims 16 characterized by the fact thatthe specific code is the combination of a spreading code and ascrambling code, by the fact that the respective lengths of thespreading code and the scrambling code are equal, and by the fact thateach sliding window contains the same blocks.
 21. Device according toany one of claims 16 characterized by the fact that the specific code isthe combination of a spreading code and a scrambling code, by the factthat the length of the spreading code is smaller than the length of thescrambling code, and by the fact that at least two consecutive slidingwindows contain different blocks.
 22. Device according to any one ofclaims 12 characterized by the fact that the CDMA wireless communicationsystem is an UTRA-TDD wireless communication system.
 23. Receiver of aCDMA wireless communication system, characterized by the fact that itcomprises a device according to any one of claims
 12. 24. Receiveraccording to claim 23, characterized by the fact that it is a cellularmobile phone.
 25. Base station of a CDMA wireless communication system,characterized by the fact that it comprises a device according to anyone of claims 12.